1. Field of the Invention
This invention relates to pressure-sensitive elements and more particularly, to piezoelectric pressure-sensitive elements which are useful in detection of a grasping force of a manipulator such as of robots and are also useful as an ultrasonic probe for medical purposes. The invention also relates to a method for making such piezoelectic pressure-sensitive elements.
2. Description of the Prior Art
In recent years, piezoelectric pressure-sensitive elements have wide utility in various fields. These elements make use of a piezoelectric effect in which when a pressure is applied to a material, dielectric polarization takes place along a certain direction, i.e. a positive charge appears at one end with a negative charge developing at the other end. The piezoelectric effect can be utilized to convert mechanical signals expressed by pressure into electric signals.
Known piezoelectric pressure-sensitive elements are, for example, those obtained by sintering a ferroelectric ceramic powder exhibiting the piezoelectric effect such as, for example, lead titanate zirconate (hereinafter abbreviated as PZT) at high temperatures, or by dispersing such a ferroelectric ceramic powder as mentioned above in a thermosetting resin such as an epoxy resin, a thermoplastic resin such as rubber or a ferroelectric resin such as polyvinylidene fluoride (PVDF), or using PVDF alone. However, the piezoelectric pressure-sensitive element using a sintered product of PZT is disadvantageous in that it is hard and brittle, so that when an impact is applied, the element is liable to break readily, thus making it difficult to make a small-size and thin element. This presents a serious problem that when such an element is used in a unit handling fine body such as a manipulator of a robot so as to detect a pressure, an object to be grasped will be broken or the element itself may break.
Accordingly, where the piezoelectric pressure-sensitive element using a sintered product such as of PZT is used in a manipulator of a robot in order to detect a pressure exerted on an object, care should be taken in such a way that an impact force is applied to the object and the pressure-sensitive element only in a degree as small as possible. This entails a complicated control program for controlling the manipulator or a slow operation speed, which is one of the serious trouble factors in the development of highly intelligent robots.
On the other hand, piezoelectric pressure-sensitive elements made of a dispersion of a ferroelectric ceramic powder such as PZT in epoxy resins or PVDF are more flexible than the pressure-sensitive element of a sintered product. However, the piezoelectric strain constant is lower than 1/2 of the constant of the element of the sintered product, presenting the problem that the pressure sensitivity is low. Moreover, a pressure-sensitive element made of a dispersion of a powder of PZT in rubber involves a similar problem as described above. These problems are considered to be ascribed to the fact that the polymer materials such as epoxy resins, rubbers and the like do not exhibit any piezoelectricity. With PVDF, the problem is raised for the following reason. If a powder such as PZT is dispersed, the resultant dispersion becomes brittle, so that a polarization treatment of the PVDF itself such as by stretching is difficult.